When propagating in a gas, acoustic waves will generate fluctuations of pressure, displacement, and temperature in the propagation medium gas. When interacting with a fixed boundary, the gas can induce conversion between acoustic energy and heat energy, which is called thermoacoustic effect.
A thermoacoustic system is an energy conversion system designed using the thermoacoustic effect principle, which may convert heat energy into acoustic energy, or convert acoustic energy into heat energy. A thermoacoustic system can be divided into two kinds: thermoacoustic engines and thermoacoustic refrigerators, where thermoacoustic engines include traveling-wave thermoacoustic engines and Stirling engines, and thermoacoustic refrigerators include traveling-wave thermoacoustic refrigerators, pulse tube refrigerators and Stirling refrigerators.
In the above thermoacoustic systems, traveling-wave thermoacoustic engines and refrigerators use inert gas, such as helium or nitrogen, as working medium. They have advantages of high efficiency, safety and long service life, thus having attracted widespread public attention. Hitherto employing a thermoacoustic engine in power generation and employing a thermoacoustic refrigerator in low-temperature refrigeration have already been successful.
Refer to FIG. 1 which is a schematic view of an existing traveling-wave thermoacoustic refrigeration system.
As shown in FIG. 1, the traveling-wave thermoacoustic refrigeration system includes three elementary units, where each elementary unit includes a linear motor 1a and a thermoacoustic conversion device 2a. 
The linear motor 1a includes a cylinder 11a, a piston 12a, a piston rod 13a, a motor housing 14a, a stator 15a, a mover 16a, and an Oxford spring 17a. 
The stator 15a is fixedly connected to the inner wall of the motor housing 14a; the mover 16a and the stator 15a are of clearance fit; the piston rod 13a and the mover 16a are fixedly connected to each other; the piston rod 13a and the Oxford spring 17a are fixedly connected to each other; during the operation of the linear motor 1a, the mover 16a, via the piston rod 13a, drives the piston 12a to perform linear reciprocating motion within the cylinder 11a. 
The thermoacoustic conversion device 2a includes a main heat exchanger 21a, a heat regenerator 22a, and a non-normal-temperature heat exchanger 23a connected in sequence. The main heat exchanger 21a is connected to a cylinder cavity of a linear motor 1a, i.e., a compression chamber 18a; the non-normal-temperature heat exchanger 23a is connected to a cylinder cavity of another linear motor 1a, i.e., an expansion chamber 19a. Thus, the thermoacoustic system constitutes a loop of medium flow.
When the traveling-wave thermoacoustic system works as a refrigerator, electrical power is supplied to the linear motor 1a. The mover 16a drives the piston 12a performing a linear reciprocating motion within the cylinder 11a, the gas medium volume within the compression chamber 18a changes, generating acoustic energy which enters into the main heat exchanger 21a, passes through the heat regenerator 22a, and most of the acoustic energy is consumed within the heat regenerator, producing refrigeration effect so as to lower the temperature of the non-normal-temperature heat exchanger. The remaining acoustic energy comes out from the non-normal-temperature heat exchanger 23a, being fed back to an expansion chamber 19a of another linear motor 1a, and then transferred to a piston 12a of the second linear motor 1a. 
When the traveling-wave thermoacoustic system works as an engine, acoustic wave absorbs heat energy and converts it into acoustic energy inside the heat regenerator 22a and the non-normal-temperature heat exchanger 23a. The acoustic energy comes out from the non-normal-temperature heat exchanger 23a after being enlarged, enters into the expansion chamber 19a of the linear motor 1a, and drives the piston 12a. The acoustic energy is divided into two parts at the piston 12a, one part enters the compression chamber 18a, being fed back into another heat regenerator 22a, another part is converted into output power through the linear motor 1a. 
During the course of study and development of the present invention, the inventors found the following technical defects of the existing traveling-wave thermoacoustic system: in the course of practical application, the non-normal-temperature heat exchanger 23a can only perform heat exchange within an extremely small temperature range. Therefore, while the traveling-wave thermoacoustic system is working as an engine, only the heat within an extremely small temperature range of the heat source supplying heat for the non-normal-temperature heat exchanger 23a can be used by the non-normal-temperature heat exchanger 23a. For example, the working temperature of the non-normal-temperature heat exchanger 23a ranges between 650° C. to 700° C., whereas the heat source and the non-normal-temperature heat exchanger 23a are exchanging heat, only within temperature range between 650° C. to 700° C., the heat can be absorbed. When the temperature of the heat source is below 650° C., the heat cannot be absorbed, thus inducing heat energy wastage and reducing conversion efficiency of the thermoacoustic energy.
In addition, while the traveling-wave thermoacoustic system is used as a refrigerator, the traveling-wave thermoacoustic system can only provide the refrigeration at one temperature, thus cannot obtain a lower refrigeration temperature. Therefore, it hampers the refrigeration performance of the traveling-wave thermoacoustic system.